What is Lamrim (known as 菩提道次第广论 or shortened as 广论)?

Venerable Ru De gives his view:

Also, from Wiki:

From Venerable Ru Jun’s discourse, I cannot help but agree with him that the Lamrim is a really precious scripture - one filled with “tips”, “hacks”, “advice”, “shortcuts” (however you call it) from past Masters. These teachings are vital to our practice!